This invention relates to a voice-band signal processor equipped with digital-to-analog and analog-to-digital converters for use in apparatus such as a full duplex modem.
Modems enable widely separated digital devices such as computers to exchange data over a telephone line or other communication line that was originally intended for voice communication. To reduce size and cost, many recent modem designs employ digital signal processing elements. A modem of these designs comprises a modulator for receiving serial bit data from a host computer and converting it to a digital voice-band signal, and converting it to serial bit data to be sent to the host computer, and a voice-band signal processor for filtering the voice-band signals and converting them between analog and digital form.
Among prior-art designs of voice-band signal processors, one design employs a programmable digital signal processor (hereinafter referred to as a DSP), a frequency converter, a digital-to-analog converter (hereinafter referred to as a D/A converter), and an analog-to-digital converter (hereinafter referred to as an A/D converter). The DSP filters the voice-band signals in digital form, restricting them to a limited frequency band suitable for the rate at which bits are transmitted and received. The D/A converter converts a filtered voice-band transmit signal from digital to analog form so that it can be placed on the communication line. The A/D converter converts an analog voice-band receive signal from the communication line to digital form, so that it can be filtered by the DSP. Both the D/A and A/D converters are oversampling devices, meaning that their sampling rate exceeds the rate at which the DSP operates. The frequency converter converts between the A/D and D/A sampling rate on the one hand and the DSP rate on the other hand by performing an interpolation or decimation process.
An advantage of this prior-art voice-band signal processor design is that it is simple and compact, and the DSP can be programmed to support a variety of communication rates. However, this design has two problems.
The first problem arises during full duplex operation when the transmit and receive sampling clock rates are not exactly matched, a situation which can easily occur due to slight differences between the crystal oscillators employed at the transmitting and receiving ends. The DSP operates at the transmit sampling clock rate, yet it must filter both the transmit and receive voice-band signals, so it processes them alternately, using half the time in each transmit sampling clock period to filter the transmit signal and half the time to filter the receive signal. If the transmit and receive clock rates are out of step, the DSP will occasionally generate receive data belonging to the wrong receive clock sampling period, causing the demodulator to receive a value differing greatly from the correct value.
The second problem arises when it is necessary to change the communication mode: to change from a rate of 300 bits per second to a rate of 1200 bits per second, for example. The filtering program of the DSP is stored in a programmable read-only memory (hereinafter referred to as a PROM). During the manufacturing process any desired program can we written in the PROM, but once the DSP has been programmed, the program cannot be altered. Accordingly, although it is easy to manufacture voice-band signal processors programmed for operation in different modes, it is not possible to change the mode afterward, which places an inconvenient restriction on the use of the voice-band signal processor.